Hydraulic alternating-stress device for material-testing units



Patented Nov. 6, 1962 fleew HYDRAULIC ALTERNATING-STRESS DEVICE FOR MATERIAL-TESTING UNITS Berthold Thiele, Dusseldorf, Germany, assiguor to Losenhausenwerk, Dusseldorfer Maschinenbau A.G., Dusseldorf-Grafenberg, Germany Filed Mar. 28, 1961, Ser. No. 98,876 Claims priority, application Germany Apr. 2, 1960 3 Claims. (Cl. 60-52) This invention relates to a hydraulic alternating-stress device for material-testing units. If a loading pump is used for the production of an alternating stress, the direction of delivery of which pump is reversible and which pump is connected through suction and pressure pipes to one cylinder each of the testing unit, the fluid is pumped eriodically alternately from one cylinder into the other. Therein, however, considerable difliculties arise, because the fluid volumes in the cylinders are often unlike or cannot be kept at the same magnitude for a long time as leakage losses cannot be avoided. In particular, such difficulties arise, if the reversible loading pump is connected to a plurality of testing cylinders at the same time through a pressure distributor. Therefore it is necessary, in such alternating-stress devices with reversible loadingpump, to provide for the possibility of replacing a lost quantity of fluid or of passing off an excess fluid quantity. The invention has for its object to provide for an automatic equalization of the fluid quantity in such an alternating-stress device.

According to this invention this is achieved in that the pump pipes alternately are connected to an equalizing pipe through a control valve actuated by the pressure difference in the pipes, said equalizing pipe being lead to the fluid sump. Thus always that pump pipe having the lower pressure is connected to the fluid sump While the fluid circulation from one cylinder to the other is maintained, so that lost fluid can be replaced out of the sump through the equalizing pipe or excess fluid can be passed olf into the sump without hindering the building up of the load pressure. Advantageously the control valve consists of a sleeve valve, one piston front face of which is exposed to the pressure in one pump pipe and the opposite piston front face of which is exposed to the pressure in the other pump pipe, and wherein always that pump pipe having the lower pressure is connected to the equalizing pipe. It is essential in such a device that the sleeve valve responds without delay also to small pressure differences in the pump pipes. For this reason, the sleeve valve advantageously is made of light metal and is made hollow in order to reduce its mass. In order to eliminate also the influence of gravity, the sleeve valve is preferably arranged in horizontal position.

An embodiment of this invention is shown schematically in FIG. 1. FIG. 2 shows a cross sectional view of the sleeve valve at an enlarged scale.

A loading pump 3 provided with a flywheel 2 is driven by an electric motor 1. The pump 3 is connected to the two cylinder chambers 7, 7" of a testing cylinder elfective in either direction through two pump pipes 4, 5 and pressure distributors 6, 6 to drive piston 7. The loading pump 3 is equipped with suitable means for controlling the directions of intake and discharge and to alternate these as required for the purposes of the invention. Such control means are well known and are therefore not shown in detail. They may include suitable valving as, for example, a slide valve, operated by an operating member 9 which, as shown schematically in FIG. 1, can be actuated by two opposing control cylinders 8, 8. These control cylinders may, in turn, be connected to be responsive to the pressures respectively, in the cylinder chambers 7', 7" with a predetermined load limit, either by direct connection of the cylinders with pipes 4 and 5, respectively, or by use of auxiliary control apparatus which translates testing cylinder pressures into operation of control cylinders 8, 3. Thus, a predetermined load pressure is produced first, for example, in the cylinder chamber 7' and, after reversal of the direction of delivery of the loading pump 3, in the cylinder-chamber 7", so that the piston of the testing cylinder exerts an alternating stress on the specimen. During one load period the fluid is pumped out of the cylinder chamber 7 and into the cylinder chamber 7", and during the second load period, the loading pump 3 sucks out of the cylinder chamber 7" and presses t e fluid into the cylinder chamber 7'. If the volumes of the cylinder chambers 7, 7" are unlike or if leakage losses occur, the loading pump 3 could not, for example suck enough fluid out of one cylinder chamber to supply the amount that would be necessary for the production of the load pressure in the other cylinder chamber (lack of fluid), or the fluid volume or one cylinder chamber may be too large to be accommodated in the second cylinder chamber (excess of fluid). Thus with these pumping processes an automatic equalization of the fluid volumes must be effected. For this purpose, a sleeve valve 10 arranged in horizontal position, is connected to the pump pipes 4, 5 in such a manner that one front face 11 is exposed to the pressure in the pipe 5 and the opposite front face 12 is exposed to the pressure in the pipe 4. One branch 5 of the pump pipes 4, 5, respectively, opens in control openings 4a, 5a of the sleeve valve 10, that opening which is on the excess pressure side being closed by the sleeve valve, and that one which is on the low pressure side being opened. An equalization pipe 14 leads to a fluid sump 15. This equalization pipe is always connected through the sleeve valve 10 with that control opening 4a or 5a which is on the low pressure side and therefore open.

If the loading pump 3 delivers fluid into the cylinder chamber 7 through pipe 5, the piston front face 11 is exposed to the greater pressure, so that the sleeve valve is moved into the position shown, wherein the control opening 5a is closed and the control opening 4a is connected with the equalization pipe 14. Thus the loading pump 3 can suck fluid both out of the cylinder chamber 7' through pipe 4 and through the equalizing pipe 14 and the branch 4', so that a possible lack of fluid is automatically compensated. If, however, with this direction of delivery of the loading pump 3 more fluid is displaced than can be sucked in by the loading pump from the cylinder chamber 7, the excess fluid flows off through the branch 4' and the equalization pipe 14 into the sump 15.

In a similar manner a fluid equalization is effected with reversed direction of delivery of the loading pump 3, where the piston front face 12 of the sleeve valve 10 is exposed to the greater pressure, and, consequently, the pipe 5, 5 is connected with the equalization pipe 14.

As the sleeve valve 10 must respond to small pressure differences without delay, the piston is made of light metal, and is provided with bores 16, 16', so that it has only a low mass inertia. An auxiliary suction pipe 18 closed by a check valve is connected with each pump pipe and is lead to the fluid sump 15. By these auxiliary suction pipes 18 the formation of a vacuum in the pump 3 or in the testing cylinder is effectively prevented, if, for example, an abnormal operating condition occurs during the starting of the unit. If, for example, the sleeve valve 10 is in the position shown, when the unit is started, whereas the loading pump 3 by chance is still adjusted in such a manner that it delivers fluid into pipe 4, a vacuum could be produced temporarily until the sleeve valve has changed over, as due to the still wrong position of the sleeve valve the fluid sucked out of the cylinder chamber is pumped into the sump 15 instead of into the cylinder chamber 7'. In order to avoid the formation of a vacuum in these cases the auxiliary suction pipes 18 are provided, through which then fluid can be sucked in through the check valve 17.

Also, an improved transition function is obtained by means of these auxiliary suction pipes 18, when the reversing piston 10 moves from one position to the other.

What I claim is:

l. A hydraulic system for driving the piston of a double-acting cylinder of an alternating-stress material testing unit, the system comprising a loading pump having suction and discharge ports, means associated with said pump for reversing the direction of delivery of said pump, a control valve, said control valve comprising a horizontally disposed sleeve valve comprising a cylinder and a freely reciprocable valve operator therein, said valve cylinder having first and second valve control ports at the respective ends thereof and first and second outlet ports arranged to be alternately opened and closed by said valve and a common inlet port arranged for connection by said valve alternately with said outlet ports, means forming a fluid sump, a pipe connecting said inlet port with said sump, first pipe means connecting a first port of said pump with a first chamber of said double-acting cylinder and with a first one of said valve outlet ports, and second pipe means connecting the second port of said pump with the second chamber of said double-acting cylinder and with the second of said valve outlet ports, the arrangement being such that in one direction of discharge of said pump said sleeve valve operator takes a position responsive to pressure at the first valve control port to close the first outlet port and direct fluid from said pump to a chamber of said double-acting cylinder and to open the second outlet port to permit said pump to suck fluid from said sump and from the other chamber of said double-acting cylinder and in the other direction of discharge of said pump said sleeve valve operator takes a position responsive to pressure at the second valve control port to close the second outlet port and direct fluid from said pump to a chamber of said double-acting cylinder and to open the first outlet port to permit said pump to suck fluid from said sump and from the other chamber of said double-acting cylinder.

2. A hydraulic system in accordance with claim 1 and including auxiliary pipe means connecting the first pipe means and the second pipe means, respectively, with the sump, each said auxiliary pipe means having a check valve therein oriented to permit only withdrawal of fluid from said sump.

3. A hydraulic system in accordance with claim 1 wherein the sleeve valve operator is composed of light weight metal and is hollow to minimize the mass thereof.

References Cited in the file of this patent UNITED STATES PATENTS l,307,839 Williams June 24, 1919 1,638,653 Cannon Aug. 9, 1927 2,750,795 Federn June 19, 1956 2,836,960 Wittren Jun 3, 1958 2,916,879 Gondek Dec. 15, 1959 2,927,429 Carlson Mar. 8, 1960 FOREIGN PATENTS 121,013 Great Britain Dec. 5, 19 8 

